The present invention relates to an aluminum nitride sintered body for use as an insulating material having a high thermal conductivity in an insulating substrate heat sink, laser tube and the like. More particularly, it relates to a composite layer aluminum nitride-base sintered body which has an improved surface electric conductivity and/or an improved wettability to metal specifically a base plate or substrate.
In recent years, the requirements for minimizing electronic instruments and improving their functions have increased prominently. As a result, semiconductors are rapidly developing towards improvements in integration density, multifunctionization, high speed, output and high responsibility, wherein more and more heat is generated by the semiconductor. Therefore, a base plate or substrate with higher heat-dissipation (or radiation) ability is needed instead of conventional Al.sub.2 O.sub.3 base plates or substrates.
Various heat engines, industrial instruments and the like are developing towards a high output, too. Accordingly materials with higher heat-dissipation are generally required nowadays.
As the materials for substrates with high heat-dissipation ability, i.e., as the materials with high thermal conductivity, there are counted, for example, diamond, CBN (cubic boron nitride), SiC (silicon carbide), BeO (beryllia), AlN (aluminum nitride), Si, and the like. However, these materials have following drawbacks. Diamond and CBN have a difficulty in manufacturing a product of such a size usable as a substrate and are very expensive. SiC cannot be used instead of an Al.sub.2 O.sub.3 substrate since SiC is semiconductive and inferior in electric characteristics such as electric insulating ability, dielectric constant and the like, compared with Al.sub.2 O.sub.3. BeO, in spite of its excellent electric characteristics, has a difficulty in its stable supply since it is not manufactured in Japan thus depends on the import from foreign contries, due to the poisonous powder occurring in the compacting process, abrasive machining process and the like.
Si has inferior electric characteristics and a low mechanical strength, and hence has a limited use as a material for the substrate. Although AlN has advantges that it is excellent in electric characteristics, for example, high insulating ability, high dielectric brake down strength, low dielectric constant, etc. and that it can be sintered under atmospheric pressure, it has a drawback that the metal layer cannot be formed on a required surface, and that a bonding strength to metal is insufficient. Thus employing aluminum nitride, therefore, multilayer substrate, laser tubes and the like have never been developed.
That is, AlN cannot be easily metallized because of a low wettability to metal, and hence it has a difficulty in use as a substrate.
For example, Japanese Patent Kokai Publication Nos. 50-75208 and 59-40404 disclose metallization after oxidizing of the surface of an AlN base plate, and Japanese Patent Kokai Publication No. 53-102310 discloses formation of a metal oxide layer on the surface of an AlN base plate before metallization.
The above-disclosed techniques, in spite of the possibility of the metallization on the AlN sintered product, have a drawback that they cannot provide sufficient adhesion or bonding strength. Further, these conventional methods have a drawback that they cannot be employed in a simultaneous sintering (so-called cofiring) for production of a multilayer structure substrate as a substrate-material.